Method for coating optical transmission glass fibers

ABSTRACT

A method and apparatus for coating optical transmission glass fibers with a resinous composition. A vessel is provided through which a glass fiber is drawn. While it is being drawn through the vessel, the fiber is spun, and a resin coating is applied. In order to maintain the air pressure within the vessel fairly constant, an exhaust vent is provided through which excess particles of the resinous composition may escape. Turbulence is thus substantially reduced preventing the fiber from swinging and allowing the particles to be sprayed smoothly. Further, the particles may be electrically charged so that they are both attracted to the optical fiber and repel one another during flight. The invention employs both of these features to produce a uniform coat on an optical transmission glass fiber.

BACKGROUND OF THE INVENTION

The present invention relates to a method for resin-coating opticaltransmission glass fibers.

Optical transmission glass fibers (hereinafter referred to as "opticalfibers") have excellent transmission characteristics of high capacityand low loss. Furthermore, a transmission line using a optical fiber hasno electrical inductance. Optical fibers are also light in weight. Dueto these and other advantages, transmission systems utilizing opticalfibers have been rapidly developed and put to practical use.

Since optical fibers are made of glass and can have a diameter as smallas 200 μm or less, mechanical strength is a very significantconsideration. In order to improve the mechanical strength of an opticalfiber cable, a tandem primary coating method has been employed in whichan optical fiber base material is melted using a resistance furnace, ahigh-frequency induction furnace, a CO₂ laser, an oxyhydrogen flame,etc. The optical fiber is thereafter drawn, and a resinous compositionimmediately coated onto the bare glass fiber and hardened. This retainsthe original strength of the glass and prevents the optical fiber frombeing damaged (see Japanese Published patent application No. 100734/76).

In applying the resinous composition coating, a die dipping method, afelt coating method, a spraying method, and the like have been used.Each method has specific advantages and disadvantages. Of these methods,the die dipping method and the spraying method are superior in that theresinous composition can be coated onto the fiber without contacting thefiber. Therefore, these two methods are used most widely. In particular,the spraying method is most suitable for high-speed coating becauseglass fibers while drawing at a high speed are subject to less influencedue to temperature. The influence of temperature is a serious problemwith the die dipping method. Furthermore, a coating film having athickness of 2-3 μm or less can only be obtained by employing thespraying method. On the other hand, the spraying method isdisadvantageous in that the thickness of the coating film obtained by asingle coating is limited, and also it is difficult to control thethickness thereof.

A conventional apparatus used in practicing the spraying method willhereinafter be explained.

In FIG. 1, an optical fiber 2 resulting from melt-drawing of a preform 1is drawn downwardly and vertically. A resinous composition is coatedonto the optical fiber 1 by a sprayer 3. The sprayer 3 may, forinstance, be a spraygun such as the Model HP-PC102 produced by OlympusCo., Ltd. of Japan.

In order to recover the excess sprayed resinous composition and toprevent it from contaminating the surrounding area, spraying is usuallyperformed in a spray coating vessel 4 which is closed except for holesthrough which the optical fiber passes. A feed unit 6 provides thesprayer 3 with the resinous composition as shown in FIG. 1.

The resinous composition is sprayed at a pressure of from 0.1 to 5kg/cm² by means of the sprayer 3. Because the spray coating vessel 4 iseffectively closed, turbulence is produced and a good spray streamcannot be obtained. This tends to cause the optical fiber to swing outof position, making it impossible to uniformly coat it with the resinouscomposition.

SUMMARY OF THE INVENTION

The present invention is directed to improvements in the prior artspraying method and provides a method which enables formation of acoating film of uniform thickness at high speeds.

While practicing the spraying method described above, it is difficult tocoat the optical fiber uniformly. However, this method possesses anadvantage in that it is possible to form a coating having a thickness aslow as 2-3 μm or less. The present invention improves upon thisconventional spraying method by providing a method which yields uniformcoating.

According to the present invention, an exhaust vent is formed in thevessel in which the resinous composition is sprayed and coated.Furthermore, a high DC voltage can be applied to the top portion of thesprayer, thus causing the sprayed resinous composition to beelectrically charged. The magnitude of the DC voltage applied to the topof the sprayer is not critical; any voltage in a range of about 5 to 60KV will suffice. The amount of electrical charge carried by theparticles of the resinous composition is one of the factors determiningthe amount of the resinous composition coated onto the optical fiber.Therefore, by changing the applied voltage, it is possible to determinethe film thickness obtained in a single coating operation.

There is no limitation on the type of resinous composition used with theinvention; any resinous composition conventionally used in the coatingof optical transmission glass fibers can be used. Preferred resinouscompositions are those containing epoxy resins, urethane resins,polyester resins, etc. which are hardened by application of heat,ultraviolet ray, electron beam, etc. Furthermore, it is desirable thatthe resin have a viscosity of 500 cps or less so that it can be sprayed.It is possible to lower the apparent viscosity of the resin bymaintaining the temperature of the coating unit or the sprayer at 60° to80° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in cross section an apparatus for resin coating opticaltransmission glass fibers according to a conventional spraying method;

FIGS. 2 and 3 illustrate in cross section two embodiments of anapparatus for resin coating optical fibers constructed in accordancewith the invention;

FIG. 4 is a schematic view showing a coating mechanism according to themethod of the invention;

FIG. 5 is a schematic diagram showing a third preferred embodiment of acoating apparatus of the invention; and

FIG. 6 is a view showing a preferred embodiment of a particular part ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereinafter be explained with reference toFIGS. 2 and 3.

In FIG. 2, a resinous composition spray coating vessel 7 is composed ofan inner casing 8 and an outer casing 9. In the center of the upper andlower sides of the casings 8 and 9 are formed optical fiber passingholes 10 and 11 through which a spinning optical fiber 2 is drawndownward and vertically. A spray hole 12 is formed through the inner andouter casings for receiving a sprayer 3.

When the resinous composition is sprayed through the nozzle of thesprayer 3, particles of the resinous composition pass across a lineconnecting the optical fiber passing holes 10 and 11 and reach the sidewall of the inner casing 8. At the point where the particles wouldotherwise reach the side wall, an exhaust vent 13 is formed in the innercasing 8. Thus, the particles of the resinous composition pass throughthe exhaust vent 13 and are recovered in the space between the innercasing 8 and the outer casing 9. While the optical fiber 2 is passingthrough the holes 10 and 11 and is drawn from the vessel 7 whilecontinuously spinning, the particles of the resinous composition sprayedfrom the nozzle of the sprayer 3 strike the optical fiber 2. Thisresults in coating the resinous composition onto the surface of theoptical fiber 2. The nonadhering particles of the resinous compositionpass through the exhaust vent 13 and disperse upon striking the innerwall of the outer casing 9. These particles are recovered in the spacebetween the inner casing 8 and the outer casing 9.

In the embodiment of FIG. 3, a spray coating vessel 7 is shown includinga casing 14, an exhaust pipe 15 connected to the casing 14 where anexhaust vent 13 is formed, and an exhaust blower 5 provided in theexhaust pipe 15. Optical fiber passing holes and a spray hole areprovided as in the conventional apparatus. When the resinous compositionis sprayed from the nozzle of the sprayer 3 through a spray hole 12,particles pass across the line connecting the optical fiber passingholes 10 and 11 and reach the side wall of the casing 14. At the pointwhere the particles of the resin composition would otherwise reach theside wall, an exhaust vent 13 is formed in the casing 14. The exhaustblower 5 is provided in the exhaust pipe 15 adjacent to the exhaust vent13. Excess resinous composition not adhering to the surface of theoptical fiber is recovered by means of the exhaust blower 5.

In accordance with the present invention, as described above, an exhaustvent is formed in the wall of a spray coating vessel at a point wherethe resinous composition sprayed from the nozzle of a sprayer wouldotherwise strike the wall. With this arrangement, the resinouscomposition spray causes less turbulence inside the coating vessel.Thus, a stable spray is obtained, the optical fiber is prevented fromswinging out of position, and a uniform resinous coating on the opticalfiber results. The method of the invention permits the stable formationof a coating which is 2-3 μm or less in thickness. In particular, theuse of a blower 5 as shown in FIG. 3 enables a more stabilized resinouscomposition spray.

In FIGS. 4-6, an optical fiber formed from an optical fiber preform isshown. The optical fiber itself is slightly positively charged due tothe thermal decomposition of glass molecules and other phenomena. It istherefore desirable that the particles of the resinous composition benegatively charged to make them adhere better. As illustratedschematically in FIG. 4, fine particles P of the resinous compositionfrom the sprayer 3 are then attracted to the fiber by the resultingCoulomb force. When fine particles of the resinous composition arecharged with the same polarity, they repel each other during flight.This prevents them from accumulating in any one region, and thusensuring the uniformity of the coating.

As shown in FIG. 5, in accordance with the invention, an optical fiber 2is drawn from a preform 1 through a spinning furnace 20. The opticalfiber 2, which is slightly positively charged, is sprayed with fineparticles P of a resinous composition by means of a sprayer 3. On thetop portion of the sprayer 3 is applied a high DC voltage by a DC source29. The optical fiber 2 having the resinous composition coating thereonis passed through a hardening furnace 24 and wound on a winding machine25. The excess particles of the resinous composition are recovered in arecovery unit 26. A vessel similar to that shown in FIGS. 1, 2 and 3 maybe used. The amount of the resinous composition deposited can beincreased by providing a grounding connection 27 to the optical fiberpreform or a grounding connection 28 to the optical fiber at the pointwhere the resinous composition is hardened after passage through thehardening furnace 24.

As illustrated in FIG. 6, it is also possible to charge the opticalfiber 2 with an electric charge of opposite polarity to that of theparticles of the resinous composition by the application of a glowdischarge, continuous corona discharge, and other methods. Experimentshave confirmed that the amount of resin deposited when the optical fiberis positively charged by application of a high DC voltage and theparticles are negatively charged is two to three times that when onlythe optical fiber is charged. It is also possible to negatively chargethe optical fiber and to positively charge the particles of the resinouscomposition.

Techniques for spraying the resinous composition according to thepresent invention include a spraying method in which a stream ofcompressed air, dry nitrogen, or similar gases is used, a method inwhich a centrifugal force is applied by rotating a disc or other device,and other methods.

We claim:
 1. An apparatus for resin-coating an optical transmissionglass fiber comprising:a vessel comprising an outer casing and an innercasing positioned within said outer casing, said inner and outer casingshaving apertures in top and bottom portions thereof for passing anoptical fiber to be coated; means for spraying a resinous compositionagainst said fiber to be coated, said spraying means being disposed atone side of said inner and outer casings, and said inner casing havingan exhaust vent provided therein on a side thereof opposite saidspraying means for passing surplus particles of said resinouscomposition.
 2. An apparatus for resin-coating an optical transmissionglass fiber comprising:a vessel having apertures in top and bottomportions thereof for passing an optical fiber to be coated; sprayingmeans for spraying a resinous composition toward said optical fiber tobe coated, said spraying means being positioned on one side of saidvessel, and said vessel having an exhaust vent formed on a side thereofopposite said spraying means; and an exhaust fan for drawing air andsurplus resinous particles through said exhaust vent.
 3. An apparatusfor resin-coating an optical transmission glass fiber comprising:meansfor spraying particles of a resinous composition toward an optical fiberto be coated; and means for charging said particles of said resinouscomposition with a polarity opposite a polarity of said optical fiber tobe coated.
 4. The apparatus of claim 3, further comprising means forspinning said fiber to be coated around a longitudinal axis thereof. 5.The apparatus of claim 4, wherein said means for charging said particlescomprises a high voltage DC source connected between a source of saidoptical fiber to be coated and said spraying means.
 6. The apparatus ofclaim 1, further comprising means for charging said particles of saidresinous composition with a polarity opposite a polarity of said opticalfiber to be coated.
 7. The apparatus of claim 2, further comprisingmeans for charging said particles of said resinous composition with apolarity opposite a polarity of said optical fiber to be coated.